Blender - Diffuse color change by color "picker" popup
Ok...at last I have figured it out! 
Probably I am not so stupid as I thought.
I made this that works perfectly:

import bpy

class MaterialColorPanel(bpy.types.Panel):
    bl_label = "3dView Material Color"
    bl_space_type = "PROPERTIES"
    bl_region_type = "WINDOW"
    bl_context = "material"

    def color_update(self, context):
        col_R=self.color_Material[0]
        col_G=self.color_Material[1]
        col_B=self.color_Material[2]
        bpy.context.object.active_material.diffuse_color=(col_R,col_G,col_B)
        print("&gt;UPDATE:", col_R, col_G, col_B)


    bpy.types.Scene.color_Material=bpy.props.FloatVectorProperty(   
                                        name="Material Color", 
                                        subtype = "COLOR_GAMMA", 
                                        default = [1.0,0.0,0.0],
                                        update=color_update)

    def draw(self, context): 
        self.layout.prop(context.scene,"color_Material")

    def invoke(self, context, event):
        print(self.colMat)
        return{"FINISHED"}


def register():
        bpy.utils.register_class(MaterialColorPanel)

def unregister():
        bpy.utils.unregister_class(MaterialColorPanel)

bpy.utils.register_module(__name__)